Aerobic treatment of solid wastes yields compost, but energy is not recovered and remains trapped in carbon content of the compost. In the process of biomethanation, the wastes are treated anaerobically to obtain energy in the form of methane gas. Biomethanation of solid wastes, particularly biodegradable wastes such as food wastes, agricultural residues and organic fractions of municipal solid wastes (MSW) is a promising technique for both, generating energy and reducing the volume of waste to be disposed of. Thus, it is realistic to consider such wastes as a renewable source for energy. Biomethanation does not require aeration and the sludge production is lower as compared to that in the aerobic processes. This further reduces the operational difficulties and costs associated with collection, transportation, storage and disposal of sludge. The solid state fermentation and anaerobic digestion are the most widely used methods for solid waste digestion.
Another reference may be made to Kaul, S. N., Khanna, P. and Nandy, T. (1994); Biogas technologies—State-of-the-art; A document prepared by National Environmental Engineering Research Institute (NEERI) for Ministry of New and Renewable Energy Sources, Govt. of India pp. 130-140; and Bodkhe, S. Y. (2003); Concept and application of a novel anaerobic reactor system for decentralized sewage treatment; Proceedings 5th GVC—Wastewater Congress, Bremen (Germany), Vol. 2, pp. 557-563 wherein, all the existing anaerobic reactor systems conventionally adopted for wastewater treatment are categorized into two major categories:                Suspended growth system (SGS)        Attached growth system (AGS)        
The drawbacks are: the reactors operated as suspended growth systems (SGS) such as most widely used upflow anaerobic sludge blanket (UASB) reactor; essentially needs granular sludge with good settling properties. At low hydraulic retention times (HRTs), increased upflow velocity of wastewater (i.e. higher hydraulic loading rate, HLR) causes biomass washout from the reactor. Reduced biomass availability for degrading increased organics at higher HLR ultimately deteriorates the performance efficiency of the reactor.
In attached growth system (AGS), e.g. anaerobic filter; porous inert packing media is provided to support the attachment of biomass. As the wastewater passes through the packed media, Suspended Solid (SS) present in the wastewater may develop progressive clogging of the packed bed and preferential hydraulic pathways may be created. This leads to short-circuiting of the movement of wastewater, which impairs the treatment efficiency of the reactor.
Reference may be made to Soli J. Arceiwala (1986); Wastewater Treatment for Pollution Control, Tata McGraw Hill Publications, N. Delhi, wherein much attention is sought towards the geometry of the UASB reactor and hydraulics for its proper functioning. A Gas-liquid-solid-separator (GLSS) is very much essential component of UASB for letting the gas to the gas collection channel at the top, while liquid rises through settler compartment and the solids settle back into the sludge zone. The upflow liquid velocity help form the sludge blanket. The drawback is that the development and maintenance of sludge blanket thickness and its retention at a particular height in reactor column for desirable period is extremely difficult and troublesome. This needs a great attention in reactor design, hydrodynamics and expertise in operation and maintenance.
Reference may be made to U.S. Pat. No. 5,670,047 by Burke; Dennis A., issued on 23 Sep. 1997, “Anaerobic treatment process for the rapid hydrolysis and conversion of organic materials to soluble and gaseous components”, wherein it is stated that, two reactors are necessary for treatment of wastewater along with a mechanical or chemical separation technique to segregate particulate constituents from partially digested influent stream. The drawback is that, two reactors require more space, power, instrumentation, process control and maintenance. Besides this, particulate separation requires chemicals & mechanical installations, which consume overheads and electricity respectively, ultimately making the treatment uneconomical. This patented technology requires hydraulic retention time of 5 days. The technology including two reactors with mechanical accessories operating at high HRT may not be acceptable for industrial applications, as it may not prove cost-effective.
Another reference may be made to U.S. Pat. No. 6,569,332 by Anisworth, J. L., Alwood, D. and Rident, T. issued on 27 May 2003, “Integrated anaerobic digester system”, wherein the digester employs pressurisable anaerobic digesters connected in parallel. The digester system is useful for biogas recovery from animal compost. The digesters needed mechanical agitation and biogas recirculation, along with digester sludge recirculation. The drawback is that, single digester vessel is not found adequate for digestion of comparatively easily biodegradable waste such as animal compost. The digester biogas is to be compressed upto 10 psi and recirculated through the digester besides digester sludge recirculation. The requirements such as compression and recirculation of biogas, recirculation of sludge and mechanical agitation within the digester need sophisticated mechanization and excessive power demand along with the expert operation and maintenance making the process less cost effective and easily adoptable.
Another reference may be made to U.S. Pat. No. 7,311,834 by Lee, Jr. and John, W. issued on 25 Dec. 2007, “Apparatus for the treatment of particulate biodegradable organic waste”, wherein the apparatus consist of thermal hydrolysis reactor operating at 130° C. and pressure at saturated water vapor pressure to produce solubilized organic material at pH above 3.15, and a separation mechanism to separate solubilized organic material and residual solids in an anaerobic reactor for digestion of solubilized organic material. The drawback is that operation and maintenance of two-phase anaerobic digestion system at higher temperature and pressure with separation mechanism need more space, spares and energy along with sophisticated mechanization ultimately rendering the treatment system less user friendly and less energy efficient. The compactness of the system is lost due to its requirement of separation mechanism.
Another reference may be made to U.S. Pat. No. 7,556,737 by Zhang, R., issued on 7 Jul. 2009, “Anaerobic phased solids (APS) digester for biogas production from organic solid wastes”, wherein a two phase anaerobic system (APS) is proposed for digestion of organic substrates to generate methane. The APS consists of a hydrolysis reactor, a buffer tank and a biogasification reactor. The drawback is that, the two-phase system has two sets of bioreactors along with a buffer tank. The operation, maintenance, space and energy consumption of two phase reactor system is higher if not double than that of the single phase reactor system.
Another reference may be made to Ortega, L., Barrington, S., Fuirt, S. R. (2008), Thermophilic adaptation of a mesophilic anaerobic sludge for food waste treatment. Journal of Environmental Management, 88, pp. 517-525, wherein UASB reactor (5 L) was operated in batch mode under the thermophilic condition (55° C.) at 10 d HRT. The drawback is that, the UASB face a serious problem of biomass washout at lower HRT and at higher OLR. The thermophilic operation is not economically attractive. The batch operation of UASB might not sustain the sludge blanket and hence the continuous operation is recommended by the authors, defeating the sole concept of UASB reactor.
Another reference may be made to Forster Carneiro, T., Perez, M., and Romero, L. I. (2008), “Influence of total solid inoculum contents on performance of anaerobic reactors treating food wastes; Bioresource Technology, 99, pp. 6994-7002, wherein, the biomethanation process of food waste was analyzed in a batch reactor with three different percentages of solids and inoculum. The best performance was observed with 20% of total solids and 30% of inoculum, which gave methane yield of 0.22 g CH4/gVS. The drawback is that, the reactor being a batch type, it cannot be used for the places where the food waste is continuously generated. The batch reactors are generally less efficient than continuous reactors.
Another reference may be made to Neves, L., Goncalo, E., Oliveira, R. and Alves, M. M. (2008), Influence of composition on the biomethanation potential of restaurant waste at mesophilic temperatures, Waste Management, 28, pp. 965-972, wherein batch degradation of restaurant waste under methanogenic conditions was carried out. The drawback is that the process has the longer time requirement of 30 days to attain not more than 85% biodegradability. The study was conducted under specific experimental/control conditions which are difficult to maintain in the field and hence the results obtained may not be warranted.
Another reference may be made to Berandino, S. D., Corta, S. and Converti, A., (2000), Semi-continuous anaerobic digestion of a food industry wastewater in an anaerobic filter, Bioresource Technology, 71, pp. 261-266, wherein, a 10 L anaerobic filter was operated at 35° C. semi-continuously with total solid concentration of 2-3 g/L and COD of 2.52 g/L at HRT of 5 d. The COD removal efficiency obtained was above 80%. The drawback is that the anaerobic filter under longer operating period and/or with higher concentration of total solids the process/system cannot be useful. Moreover, the system cannot be useful for the treatment of slurries, the form in which food waste is generally disposed of. Under high solid concentration and longer period of operation, the anaerobic filter gets clogged and the overall efficiency of the reactor deteriorates. The backwash required to remove clogging needs water and energy besides additional mechanization.
Another reference may be made to He, Y., Xu, P., Li, C., and Zhang, B. (2005), High concentration food wastewater treatment by an anaerobic membrane bioreactor, Water Research, 39, pp. 4110-4118, wherein, ultrafiltration membrane bioreactor was used for treatment of high strength food wastewater. The drawback is that significant flux decline is caused by the formation of a thick biofouling layer. The membrane bioreactor needs expertise to operate, chemicals for washings and the process is not energy efficient and hence cannot be useful for the treatment of huge amount of food waste slurries.
Another reference may be made to R. R. Singhania, A. K. Patel, C. R. Soccol, A. Pandey (2009), Recent advances in solid-state fermentation, Biochemical Engineering Journal, 44(1), pp. 13-18 wherein the solid state fermentation of biodegradable wastes has been found to be economically feasible. The drawbacks are: it faces the challenges such as difficulties on scale-up, poor mixing and hence the low mass transfer rates, weak control of process parameters (pH, heat, moisture, nutrient conditions, etc.), problems with heat build-up, higher impurity product, increasing recovery product costs
Another reference may be made to Jong Ik Park, Yeoung-Sang Yun, Jong Moon Park (2002), Long-term operation of slurry bioreactor for decomposition of food wastes, Bioresource Technology, 84, pp. 101-104 wherein aerobic digestion of food waste was carried out in an 80 L stirred tank reactor (50 L working volume) used as a slurry bioreactor. The reactor was equipped with a pitched blade turbine-type impeller and sparger for agitation and aeration, respectively. The reactor was operated with a waste addition rate of 3 g dw/L/day, an agitation speed of 120 rpm and aeration rate of 50 L/min. Using data for time variation of dissolved oxygen, the oxygen requirement for decomposition of food wastes was estimated to be 5.0 g O2 g−1 dry weight of food wastes. During operation for 90 days, 91% reduction of food wastes was achieved. The drawback is that the aerobic slurry bioreactor consumes dissolved oxygen along with a mechanism to supply and mix oxygen with food slurry and energy to carry out these operations. Thus, the aerobic treatment of food waste is energy intensive, need mechanical installations and expert O&M.
Another reference may be drawn to US Patent No. US2004/002571A1 by Bone, T., Pedersen, L. J., issued on 12 Feb. 2004, “Concept for slurry separation and biogas production”, wherein anaerobic digestion of animal composts, energy crops and other similar organic substrates has been considered for biogas generation. The drawback is that, the patent discusses only the concept of biomass slurry preparation and does not specify any reactor technology to be exploited for biogas production.
Another reference may be drawn to Xu, S. Y., Lam, H. P., Karthikeyan, O. P., Wong, J. W. C. (2011), optimization of food waste hydrolysis in leach bed coupled with methanogenic reactor: effect of pH and bulking agent, Bioresource Technology, 102(4) pp. 3702-3708, wherein, leach bed reactor (LBR) was coupled with methanogenic UASB reactor, for the treatment of food waste. The drawback is that it needed operation and maintenance of two reactors with additional requirement of leachate recirculation. The LBR needed bulking agents or co-substrates for methanation. The strong control over acidogenesis was essentially recommended. The use of bulking agents affected the working volume of the reactor. The reduced reactor volume increases the cost of the treatment.
Another reference may be drawn to Bernstad, A., Davidsson, A., Tasi, J., Persson, E. Bissmant, M. Jansen, J. Lacour (2013), Tank connected food waste disposer systems—current status and potential improvements, Waste Management, 33, pp. 193-203, wherein, unconventional system for separate collection of ground food waste and its settling was carried out for methane generation. The drawback is that, the system is operated on the principles of two-phase anaerobic process inheriting their drawbacks as pointed out earlier. The system comprised of disposer and settling tanks. The fugitive emissions of methane from the settling tanks were also observed. Such fugitive emissions reduce the inventory of the biogas and cause, on the contrary, the greenhouse effect.
Another reference may be drawn to Ratanatamskul, C., Onnum, G., Yamamoto K. (2014), A prototype single-stage anaerobic digester for co-digestion of food waste and sewage sludge from high-rise building for on-site biogas production, International Biodeterioration & Biodegradation, 95, Part A, pp. 176-180, wherein, co-digestion of food waste (FW) and sewage sludge (SWS) was carried out with the help of a prototype single-stage anaerobic digester. The prototype system was efficiently producing methane at hydraulic retention time (HRT) of 27 days corresponding to organic loading rates of 7.9 kgCOD/m3.d. The feed mixed waste ratio (FW/SWS) of 10:1 by weight was selected. The drawback is that, the system uses a co-digestion technique thereby curtailing the capacity of the system to treat food waste. The system is operated at the long HRT of 27 days for satisfactory generation of methane. Reactor operation at long HRT reduces cost-effectiveness of the treatment system.
Another reference may be made to U.S. Pat. No. 9,005,443, 14 Apr. 2015 by Arnoldsen, Jr.; Ronald E. Arnoldsen; Debra A. “Compartmentalized anaerobic digesters” wherein an anaerobic digestion device includes a digester body configured to receive organic waste and a plurality of plates coupled to one another so as to divide an interior volume of the digester body into a plurality of compartmentalized chambers. The compartmentalized chambers are movable relative to the digester body to advance slurry of said organic waste along a length of the digester body. A plurality of ports spaced along the digester body and arranged to vent biogas from the digester body. A storage vessel is configured to receive and store biogas received from the digester body via the ports, and a heating system configured to heat the digester body. The heating system is fuelled by the biogas vented from the digester body. The drawback is that the overall configuration of the system is extremely complicated in construction and operation and maintenance. The system needs a heating system to operate it at optimum conditions. In some exemplary aspects, a need of water containing an anti-freezing solvent may be heated by the water heater and then circulated through one or more flow lines. An insulator may be placed about the hot water tubing and digester body for efficiency. Hence, it may not be a cost-effective way of treatment of wastes.
Another reference may be made to S. J. Grimberg, D. Hilderbrandt, M. Kinnunen, S. Rogers (2015), “Anaerobic digestion of food waste through the operation of a mesophilic two-phase pilot scale digester—Assessment of variable loadings on system performance” Bioresource Technology, Volume 178, Pages 226-229 wherein, Single and two-phase operations were compared at mesophilic operating conditions using a digester system consisting of three 5-m3 reactors treating food waste generated daily within the university campus kitchens. When normalizing the methane production to the daily feedstock characteristics, significantly greater methane was produced during two-phase mesophilic digestion compared to the single-stage operation (methane yield of 380 vs 446-L CH4 kg VS−1; 359 vs 481-L CH4 kg COD−1 removed for single vs two stage operation). The fermentation reactor could be maintained reliably even under very low loading rates (0.79±0.16 kg COD m−3 d−1) maintaining a steady state pH of 5.2. The drawback is that despite the potential difficulties with operating the more complex system, the two-stage process could be successfully maintained through low-loading periods as are typically experienced during summer months on a university campus. Therefore, it is usual that the system has experienced many overloading upsets. No significant difference was observed in the effluent concentrations of single-stage vs two-stage operation. There was no significant difference in the average total methane produced per day between the single-stage and two-stage system. The pH of fermentation reactor goes down up to 5.2, whereas the pH at methanogenic reactor has to be maintained at above 7.0 for its optimum operation by addition of alkali. This increases the cost of operation and maintenance.
Another reference may be made to U.S. Pat. No. 7,320,753, Jan. 22, 2008 by Kurt Frederich Roos, “Anaerobic digester system for animal waste stabilization and biogas recovery”, wherein, an anaerobic digester system comprising a substantially flexible bladder for anaerobically digesting animal waste, with biogas production and recovery is given. The substantially flexible bladder has one or more waste inlets, digester effluent outlets, sludge access ports, and biogas outlets on a top surface thereof. The bladder and the one or more biogas storage containers may be constructed with reinforced geo-membrane material. The bladder may include an internal baffle defining a U-shaped interior having an inlet side and an outlet side. The bladder, for primary waste treatment, may be complemented by other structures for secondary and tertiary waste treatments. As biogas is produced inside the bladder, the waste is pushed out of the digester effluent outlet into the external displacement tank and when biogas is used, the displaced waste flows back into the bladder through the digester effluent outlet. The drawback is that the construction of the digestor and biogas collection system is very complicated and the operation demands expertise. The material of construction of substantially flexible bladder is essentially reinforced geo-membrane. The cost of construction is high and only specific material is to be used for construction. The bladder, for primary waste treatment, may be complemented by other structures for secondary and tertiary waste treatments.
Another reference may be made to US Patent No. 0130290A1 Jun. 16, 2005 by Choate, Chris E.; Sherman, Paul A. “Organic waste material treatment system”, wherein, a system for treating organic waste material, via a multi-stage process involving anaerobic hydrolysis, anaerobic digestion of the liquid hydrolysis product, and aerobic composting of the solids remaining after hydrolysis is given. The organic waste materials may be pre-treated by adding a amount of liquid inoculant sufficient to raise the moisture content of the organic waste to a minimum of 60%. The organic waste material is then placed within a sealed hydrolysis vessel, which may take the form of a cylindrical polymer bag. The liquid hydrolysis product transferred from the vessel, which may be temporarily stored in a holding tank, is passed to a conventional anaerobic digester. In a thermophilic digester, methanogenic bacteria convert organic matter that is dissolved and/or suspended in the liquid hydrolysis product to a biogas product. The drawback is that a multi-stage process is involved in the process of digestion, which include, pre-treatment of waste, anaerobic hydrolysis, anaerobic digestion of the liquid hydrolysis product, and aerobic composting of the solids remaining after hydrolysis. The basic reactor used is a conventional thermophillic anaerobic digester. This means that there would be no benefit on the rate of biogas production. Rather, the treatment sceme is very lengthy and requires many reactors.
Another reference may be made to A. Ahamed, C. L. Chen, R. Rajagopal, D. Wu, Y. Mao, I. J. R. Ho, J. W. Lim, J.-Y. Wang (2015), Multi-phased anaerobic baffled reactor treating food waste, Bioresource Technology, Volume 182, Pages 239-244, wherein, the study was conducted to identify the performance of a multi-phased anaerobic baffled reactor (MP-ABR) with food waste (FW) as the substrate for biogas production and thereby to promote an efficient energy recovery and treatment method for the wastes with high organic solid content through phase separation. A four-chambered ABR was operated at an HRT of 30 days with an OLR of 0.5-1.0 g-VS/L d for a period of 175 days at 35±1° C. Consistent overall removal efficiencies of 85.3% (COD), 94.5% (CODs), 89.6% (VFA) and 86.4% (VS) were observed throughout the experiment displaying a great potential to treat FW. Biogas generated was 215.57 mL/g-VSremoved d. Phase separation was observed and supported by the COD and VFA trends, and an efficient recovery of bioenergy from FW was achieved. The drawback is that considering the total reactor volume of MP-ABR in this study, the biogas production is relatively low which is 88.45 mL/L of the reactor volume. The reactor was operated in two operational periods with a modification to the reactor configuration in the second period. This is not feasible in actual practise. To prove the feasibility, pilot scale systems should be operated. Each compartment was provided with an overhead mixer rotating at a speed of 100-150 rpm to minimize dead zones and short circuiting. Basically, ABR is not meant for treatment of food waste. The dead zones and short circuiting impair the performance. Overhead mixers create chances of biogas leakages.
The publications and patents cited above on anaerobic treatment/biomethanation of organic wastes/substrates indicated the process of slurry preparation/feed to the anaerobic digestion in general and the operation of anaerobic reactor/anaerobic digestion apparatus in particular. The apparatus/devices used for biomethanation of wastes are conventional reactors; used as a methanation phase/reactor in a two phase biomethanation/digestion system.
The operation and maintenance (O&M) of the two-phase digestion (acidogenesis and methanaogenesis) systems consumes space, energy, chemicals to yield the efficient biomethanation potential. Moreover, the conventional reactors used for methanogenesis part carry inherent demerits. These aspects together render the existing systems techno-economically unviable for biomethanation of biodegradable wastes/slurries. Hence, the present invention proposes a single reactor for biomethanation of wastes at mesophilic temperatures.